Singlet Molecular Oxygen Reaction

It is known that photochemistry is the study of chemical changes produced by electronically excited species. Most of the photochemical transformations come from triplet excited state, due to its higher life-time. But oxygen molecule is a rare case, its ground state is a triplet while reactive state is electronically excited singlet state. The life time of electronically excited triplet state is relatively low, so significantly populated is singlet excited state. 

The first excited state of electronically promoted molecular oxygen behave as an electrophile since it has two paired electrons while second excited state behave like radical. The singlet excited state of oxygen molecule is most stable state. Most of the photooxygenation reactions are shown by the singlet molecular oxygen.  

The most acceptable method for the generation of singlet molecular oxygen, is the direct irradiation of oxygen molecule in presence of a sensitiser. The photoexcitation of ground state molecule is carried out in the reaction mixture. 

Another method for the generation of excited singlet molecular oxygen is by the oxidation of hydrogen peroxide with other oxidants like sodium hypochloride, alkaline solution of bromide or alkaline solution of peracids as given above. 

Oxidation caused by singlet molecular oxygen is known as photosensitised oxygenation or photochemical oxygenation. The photo-oxygenation is not similar for all substances and is affected by the use of sensitiser or altering reaction conditions. It is well understood by the following examples  

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Singlet molecular oxygen ( A is an electron acceptor powerful enough to react with olefins in the pseudoexcitation band. The [2h-2] cycloaddition and ene reactions and the stereoselectivities are reviewed in this subsection. 

After 28 years the perepoxide quasi-intermediate was supported by a two-step no intermediate mechanism [71, 72]. The minimum energy path on the potential energy surface of the reaction between singlet molecular oxygen ( A and dg-teramethylethylene reaches a vaUey-ridge inflection point and then bifurcates leading to the two final products [73]. 

The concept, quasi-intennediate [70], was introduced in 1975 to symbolize a boundary between concerted and stepwise mechanisms. Recent advances in computer chemistry are allowing us to investigate subtle problems more clearly in the years since 2000. Concerted/stepwise boundary mechanisms were proposed for other diverse reactions than those of singlet molecular oxygen ( A. ... 

Following the discovery of the ene reaction of singlet molecular oxygen ( Ap (Scheme 15) in 1953 by Schenck [88], this fascinating reaction continues to receive considerable mechanistic attention today. The importance of a path via the perepoxide intermediate or a perepoxide-Iike transition state [13] or the perepoxide quasi-intermediate [70] was proposed for the ene reactions of singlet oxygen with alkenes affording allylic hydroperoxides. 

On the other hand. Type II process competes efficiently with the electron-transfer pathway in aerobic environments where the concentration of ground triplet state molecular oxygen is relatively high ( 0.27 mM), and singlet molecular oxygen (1O2) is the most abimdant ROS generated under these conditions, with a quantum yield 0.48 (Valle et al., 2011), eqn. 8. It is also possible an electron-transfer reaction from 3RF to 02 to form anion superoxide, but this reaction occurs with very low efficiency <0.1% (Lu et al., 2000). 

SCHEME 1. [4 + 2], [2 + 2] and ene addition reactions of singlet molecular oxygen to alkenes... 

In 1963, E. J. Bowen published his classic review The Photochemistry of Aromatic Hydrocarbon Solutions, in which he described two major reaction pathways for PAHs irradiated in organic solvents photodimerization and photooxidation mediated by the addition of singlet molecular oxygen, 02 ) (or simply 102), to a PAH (e.g., anthracene). For details on the spectroscopy and photochemistry of this lowest electronically excited singlet state of molecular oxygen, see Chapter 4.A, the monograph by Wayne (1988), and his review article (1994). For compilations of quantum yields of formation and of rate constants for the decay and reactions of 02( A), see Wilkinson et al., 1993 and 1995, respectively. 

Singlet molecular oxygen, Oj, is also thought to arise, perhaps in reaction 4 or 5 (in place of dioxygen). The addition of 02 completes the cast of characters comprising molecules which may mediate the effects of O , since whenever O is formed, H2O2, OH and O2 all may exist in aqueous solutions in the presence of micromolar concentrations of ionic iron which contaminate many buffers. Armed with the capacity to form species with such a variety of affinities for electrons, the PMN is endowed with the ability to initiate a formidable array of chemical reactions, not merely between the various species themselves but also with its own constitutents and those of ingested microbes. 

It has been suggested23,59 that ground-state atomic oxygen, produced in discharges or thermal reactions, can react with ozone to produce a singlet molecular oxygen product... 

Of the many alternative mechanisms which have been proposed for the excitation of singlet molecular oxygen, only one has been shown by laboratory studies to give excited oxygen (see Sect. IV). Young and Black59 have found that the reaction... 

It is now important to see whether singlet molecular oxygen can take part in any processes which cause oxidation of nitric oxide to nitrogen dioxide. The direct reaction... 

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